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Photography 1, 2 & 3: Camera Exposure Controls

Camera Exposure Controls

ISO  Unlike film cameras, a digital camera can change its ISO, or light sensitivity, for each individual exposure.  For the best quality, use the lowest ISO setting possible for the existing light conditions.

LIGHT METER Meters can be built into your camera or they can be a separate unit. Camera meters vary.  Read your camera manual to learn how to operate yours properly.  Your meter indicates an exposure which averages the scene's brightness ranges to give you a gray picture.  This is usually acceptable because most scenes have an average range of tones.  However, if your scene is not average, you must compensate with your exposure settings.  If your scene is predominately bright tones, overexpose by one half to one full stop.  If the scene is predominately dark tones, underexpose by one half to one full stop.  It is always a good idea to bracket your exposure.

BRACKETING You can insure a properly exposed image by bracketing.  Take three shots:  one under-exposed one half to one stop; one according to your light meter; and one overexposed by one half to one full stop.  Most digital cameras have a bracketing feature.

SHUTTER SPEEDS This camera exposure control is located in various positions depending upon the camera.  However, the view screen almost always displays the chosen setting.  Shutter speeds can be set manually or by using the "shutter speed priority mode."  In this mode, the photographer selects the shutter speed and the camera automatically selects the appropriate aperture.  Common shutter speeds are:

B - 1 - 2 - 4 - 8 - 15 - 30 - 60 - 125 - 250 - 500 - 1000

These numbers represent denominators of fractions of seconds; 1/8, 1/15, 1/1000, etc.  Therefore, the bigger the number, the faster the shutter speed and the less the amount of light that reaches the film.  Faster speeds are used to stop subject motion as well as camera movement.  Slow speeds are used to show subject motion or are used for time exposures.  Time exposures are exposures longer than speeds marked on the shutter speed dial.  Set the dial at "B" for bulb.  The shutter will remain open as long as the sutter release button remains depressed.  You should use a tripod and a shutter release cable.  Remember shutter speeds control motion.

APERTURE Aperture is the size of the opening in the len's shutter.  This camera exposure control is located in various positions depending upon the camera.  However, the view screen almost always displays the chosen setting.  Apertures can be set manually or by using the "aperture priority mode."  In this mode, the photographer selects the aperture and the camera automatically selects the appropriate shutter speed.  Some common apertures are:

32 - 22 - 16 - 11 - 8 - 5.6 - 4 - 2.8 - 2 - 1.4 - 1    

The numbers (like shutter speeds) represent denominators of fractions:  1/32, 1/22, 1/16, etc.  Therefore, the larger the number, the smaller the shutter opening.  This fraction represents the size of the shutter opening in relation to the focal length of the lens.  Focal length is the distance between the optical center of the lens to the film plane when the lens is focused to infinity.  For example, if a lens' focal length is 50mm, then its f/8 opening woudl 50/8 = 6.25mm.  Apertures also allow the photographer to control depth of field. The largest aperture on a lens is used to designate the "speed" of the lens because its larger opening lets light enter the film fastest.  So a lens whose largest aperture is 2.8 is referred to as an F/2.8 lens.  Faster lenses are usually more expensive because their larger apertures require larger lenses to accommodate them.

DEPTH OF FIELD This could be called "range of focus" because it refers to the range of the scene that will be sharp at a given aperture:  the nearest area in front of the point of focus that will be sharp to the fartherest area behind the point of focus that will be sharp.  The smaller the aperture that is used, the more depth of field is in the photo.  A large depth of field is desirable for subjects such as landscapes while a small depth of field is good for subjects such as portraits.

EQUIVALENT EXPOSURES In photography, the f/stop conceptually refers to the doubling or halving of exposure.  This can be done by changing the aperture, the shutter speed or ISO you use.  For example:  F/8 lets half as much light expose the film as f/5.6, but twice as much as f/11.  A shutter speed of 125 lets half as much light expose the film as 60, but twice as much as 250.  ISO 200 is one stop more sensitive tha ISO 100, but one stop less sensitive than ISO 400.  When the ISO number doubles it indicates an increase of one f/stop in sensitivity.  So, if your light meter indicates a correct exposure for ISO 200 film of f/8 at 125 shutter speed, then equivalent exposures would be:

ISO 200 - f/5.6 - 250 and ISO 200 - f/11 - 60 and ISO 100 - f/5.6 - 125      

The ability to choose from among equivalent exposures is what allows for creativity.  It lets the photographer choose what is most important:  to control motion, depth of field, or the amount of grain.